1. Field of the Invention
The present invention relates to a method for depositing solid electrolyte layers used in fuel cells and the like.
2. Description of Related Art
A fuel cell using a solid electrolyte has a basic structure which includes an anode, an electrolyte layer, and a cathode, and generates electricity in accordance with the following principle.
At a temperature of approximately 1,000.degree. C., a fuel gas such as hydrogen is fed into the anode and an oxidation gas such as air is fed into the cathode so that oxygen in the oxidation gas is electrochemically reduced to negative ions at the interface between the cathode and the electrolyte. The oxygen ions pass through the electrolyte layer toward the anode by means of a difference in oxygen partial pressure between two sides of the electrolyte layer as the driving force, and are electrochemically oxidized by a reaction with the fuel gas at the anode. If the anode and the cathode are electrically connected at this stage, electricity can be obtained.
In such a solid electrolyte fuel cell, since higher generation efficiency can be obtained as resistance of the electrolyte layer decreases, it is desirable that the electrolyte layer be as thin as possible. The layer will easily break, however, if the thickness of the layer is decreased, and thus,the electrolyte layer is generally deposited at a thickness of several tens of microns to several hundreds of microns on an anode or a cathode as a substrate in order to ensure sufficient mechanical strength.
Also, if the electrolyte layer is porous and is not sufficiently dense for the gas used, oxygen gas passes through the electrolyte layer without being ionized, resulting in significant deterioration in the generation efficiency of the fuel cell. Thus, it is also desirable that the electrolyte layer be dense.
There are various methods for depositing a solid electrolyte layer on an electrode substrate, such as a chemical vapor deposition-electrochemical vapor deposition (CVD-EVD) process, a slurry dipping process, an electrophoretic deposition process, and a thermal spraying process by plasma-jet or the like.
In the CVD-EVD process, as disclosed in U.S. Pat. No. 4,609,562, an electrolyte layer is formed by reacting a metal compound gas with an oxidizing gas such as water vapor or oxygen at high temperatures. A dense layer having a thickness of several tens of microns can be deposited in this process; however, the yield is significantly low since the raw materials are supplied in a gas phase, and productivity is also significantly low because of very slow deposition rates.
In the slurry dipping process and the thermal spraying process, zirconia particles or the like as a material are deposited on a substrate to form an electrolyte layer. Although the deposition rate in these processes is significantly higher than that in the CVD-EVD process, the layer tends to be porous, and thus it is difficult to form a completely dense electrolyte layer on an electrode substrate for a fuel cell. Also, in the electrophoretic deposition process, electrolyte particles having a given zeta electric potential migrate by means of a potential gradient generated in the electric cell so that they are deposited on the substrate. Although this process has a high deposition rate and is excellent in forming a dense layer, as disclosed in U.S. Pat. No. 5,700,361, repeated deposition is required in order to form a completely dense electrolyte layer on an electrode substrate for a fuel cell, resulting in significantly low productivity.
Accordingly, in order to form a dense electrolyte layer without significantly decreasing productivity, two-stage deposition methods have been disclosed in, for example, Japanese Patent Publications Nos. 62-268063, 63-164174, and 63-285877, in which, an electrolyte layer is deposited nearly up to a desired thickness by the slurry dipping process or the thermal spraying process having a fast deposition rate, and then, the same electrolyte is further deposited on the porous layer for a short period of time to achieve the desired thickness and to densify by sealing the layer by a CVD process, the CVD-EVD process, or a physical vapor deposition (PVD) process.
In accordance with the slurry dipping process, however, a binder is used in order to increase viscosity of the slurry of electrolyte particles. Since pores are produced by the binder remaining in the layer, sealing cannot be completely performed by the subsequent CVD-EVD process, and thus a sufficiently dense layer cannot be formed. Also, it is difficult to obtain a uniform thickness because of dripping during deposition.
In accordance with the thermal spraying process, deposition is performed by spraying particles, which are molten at high temperatures, onto a substrate at high speed, by means of deformation of the particles during collision. However, because of the nature of the process, closed pores remain, and thus, sealing cannot be completely performed by the subsequent CVD-EVD process. Accordingly, a sufficiently dense layer cannot be formed, the same as in the slurry dipping process. Also, since the thickness and density vary depending on the spraying duration and the spraying angle, it is difficult to control the thickness and density of a electrolyte layer having a complex and wide area.